Marks [1] has shown that the power consumption of non-multiplexed displays can be reduced to about 50% when the charge across the pixels is discharged by shorting the two electrodes for a short time interval before charging the pixels to a voltage with opposite polarity. He has also analyzed and estimated the power consumption in multiplexed LCDs [2], based on the model of the matrix display shown in FIG. 1. Each pixel in the LCD is represented as a capacitor. One electrode of the pixel is connected to a row address line and the other electrode is connected to a column address line. Capacitance of pixel depends on the state of the pixel since the effective dielectric constant is determined by the orientation of the liquid crystal molecules. The capacitance of the ON pixel Con is at least twice the capacitance of the OFF pixel Coff in nematic liquid crystal displays because the dielectric constant of the rod-like liquid crystal molecules is higher when measured parallel to its long axis as compared to the other two perpendicular directions.
Display drivers are used to apply the waveforms to the rows and columns of the matrix display. Power consumption of the panel is the power dissipated in the resistors while charging and discharging the pixels to voltages as dictated by the addressing technique. Marks [2] has estimated the power consumption in a matrix display driven by the conventional line-by-line addressing when the worst-case pattern that consists of alternate ON and OFF pixels is displayed. He has shown that the power consumed by the multiplexed display is proportional to N2M. Here, N is the number of lines multiplexed and M is the number of columns in the matrix display. This analysis is restricted to just one polarity inversion per frame. Frequent polarity reversal is introduced in the addressing waveforms to improve the brightness uniformity of the display. It induces transitions in places where there were no transitions and suppresses transitions in some other places. Polarity of the addressing waveforms is changed after scanning few address lines in most of the passive matrix LCDs. We have extended the analysis of power consumption in the line-by-line addressing technique by including polarity inversion as an additional parameter.
Power is dissipated in the drive circuit when pixels in the passive matrix displays are charged and discharged. Substituting the select pulses in the scanning waveforms with multi-step waveforms will reduce the power dissipation. The rows in the matrix displays are selected with a pulse because they are easy to generate.
Analysis of Line-by-Line Addressing with Multiple Polarity Inversions in a Frame
Let Vr and Vc be the amplitudes of the row and column voltages. Let np be the number of polarity inversions in a frame (Marks had assumed np=1 in his analysis) and f be the frame frequency of the line-by-line addressing. Let Con and Coff be the capacitance of the pixels in ON and OFF states respectively. Table I gives the voltage transition across the pixels based on the two neighboring pixels in a column when the row (i) is unselected and the row (i+1) is selected. These transitions depend on the state of the pixels in rows (i) and (i+1) as well as the polarity inversion. The voltage transitions when a polarity inversion is introduced are shown within the parentheses.
TABLE IVOLTAGE TRANSITIONS ACROSS PIXELSIN LINE-BY-LINE ADDRESSINGState of theVoltage swing acrosspixel inthe pixels inRowrowrowother(i)(i + 1)row (i)(i + 1)rowsONONVrVr0(Vr + 2 Vc)(Vr + 2 Vc)(2 Vc)ONOFFVr + 2 VcVr − 2 Vc2 Vc(Vr)(Vr)(0)OFFONVr − 2 VcVr + 2 Vc2 Vc(Vr)(Vr)(0)OFFOFFVrVr0(Vr − 2 Vc)(Vr − 2 Vc)(2 Vc)
Case 1: Power consumed by a blank screen when all the pixels are OFF. Power consumed in a column during a transition i.e., when the row (i+1) is selected and polarity of the voltages applied to the two rows remains unchanged is as follows.
                              P                      tran            .                          =                                                                              C                  off                                ⁢                                  V                  r                  2                                            2                        +                                                            C                  off                                ⁢                                  V                  r                  2                                            2                        +                                          (                                  N                  -                  2                                )                            ⁢                                                C                  off                                ⁡                                  (                  0                  )                                                              =                                    C              off                        ⁢                          V              r                              2                ⁢                                                                                                                          (        1        )            
The first term corresponds to the power dissipated while discharging the pixel in row (i) from Vr−Vc to −Vc and the second term corresponds to the charging the pixels in row (i+1) from −Vc to Vr−Vc while the third term corresponds to the rest of the (N−2) pixels in a column without any change in the voltage across them. Similarly, the power dissipated when the polarity of the select voltage changes is given in (2).
                              P                      tran            .                    ′                =                                                                              C                  off                                ⁡                                  (                                                            V                      r                                        -                                          2                      ⁢                                              V                        c                                                                              )                                            2                        2                    +                                                                      C                  off                                ⁡                                  (                                                            V                      r                                        -                                          2                      ⁢                                              V                        c                                                                              )                                            2                        2                    +                                    (                              N                -                2                            )                        ⁢                                                                                C                    off                                    ⁡                                      (                                          2                      ⁢                                              V                        c                                                                                                                                        )                                                  2                            2                                                          (        2        )            
Power consumption in a column during a frame is given byPcolumn(frame)=(N−np)Ptran.+npP′tran.  (3)
Power consumed by the whole display panel is obtained by multiplying (3) by M, the number of columns in the display and f, the frame frequency as shown in the following equation.PALL—OFF=MCoffVc2(N2+np(2N−4√{square root over (N)}))f  (4)
Case 2: Power consumed by a blank screen, when all the pixels are ON is given in the following expression.PALL—ON=MConVc2(N2+np(2N+4√{square root over (N)}))f  (5)
Case 3: Power consumed when a checkerboard pattern is displayed is given in (6). Here, the number of pixels in ON and OFF states are equal and the neighboring pixels in the vertical as well as the horizontal direction are in the opposite states.
                              P                      ON            ⁢            _            ⁢            OFF                          =                                                            MV                c                2                            2                        ⁡                          [                                                                                                                                            C                          on                                                ⁡                                                  (                                                                                    3                              ⁢                                                              N                                2                                                                                      +                                                          4                              ⁢                              N                              ⁢                                                              N                                                                                      -                                                                                          n                                p                                                            ⁡                                                              (                                                                                                      2                                    ⁢                                    N                                                                    +                                                                      4                                    ⁢                                                                          N                                                                                                                                      )                                                                                                              )                                                                    +                                                                                                                                                          C                        off                                            ⁡                                              (                                                                              3                            ⁢                                                          N                              2                                                                                -                                                      4                            ⁢                            N                            ⁢                                                          N                                                                                -                                                                                    n                              p                                                        ⁡                                                          (                                                                                                2                                  ⁢                                  N                                                                -                                                                  4                                  ⁢                                                                      N                                                                                                                              )                                                                                                      )                                                                                                        ]                                ⁢          f                                    (        6        )            
We have also introduced duty cycle in the pulses of the line-by-line addressing technique. Power consumption after the inclusion of duty cycle is analyzed and compared in the next section.